In general, a floating support has anchor means to enable it to remain in position in spite of the effects of current, wind, and swell. It generally also includes means for storing and processing oil and off-loading means for use with off-loading tankers, where such tankers call at regular intervals to remove the production. Such floating supports are commonly referred to as floating production storage off-loading (FPSO) units.
Floating supports are:                either of constant heading type, i.e. they possess a plurality of anchors, generally situated at each of the corners of said floating support and serving to keep it on a heading that cannot vary, leaving it free to move only in roll and in pitching and limiting any movement in surge and yaw;        or else of the turret type, i.e. all of the anchors converge on a cylindrical structure secured to the vessel, but free to rotate about a vertical axis ZZ′, thus leaving the floating support free to turn around said turret and position itself in the direction of least resistance for the resultant of the effects of wind, current, and swell on the floating support and its super-structures.        
The floating support is thus either anchored at its four corners so that it retains a heading that is substantially constant throughout the lifetime of the installations, or else it is anchored at a single point referred to as a “turret” that is generally situated towards the front of the vessel, generally in the front third, or indeed outside the vessel several meters from the stem of the vessel. The FPSO then swings about its turret and naturally takes up a position in the direction of least resistance relative to the forces created by swell, wind, and current. The bottom-to-surface connections are connected to the internal portion of the turret that is substantially stationary relative to the earth and rotary joints known to the person skilled in the art serve to transfer fluids to the FPSO together with electrical power or electric signals between said bottom-to-surface connections and said FPSO. Thus, for an FPSO on a turret, the FPSO can swing through 360° around the axis of its turret, which itself remains substantially stationary relative to the earth.
When conditions are severe, or indeed extreme as in the North Sea, an advantageous floating support is of the turret type in which all of the bottom-to-surface connections converge on a turret prior to reaching the FPSO proper, via a rotary joint coupling situated on the axis of said turret. In general, the pipes of bottom-to-surface connections are constituted by flexible pipes directly connecting the pipes that rest on the sea bed to the turret, with said flexible pipes generally being organized radially or in a star configuration in a uniform distribution around the axis of said turret. That type of bottom-to-surface connection is more particularly for use in depths in the range 200 meters (m) to 750 m.
The present invention relates more particularly to a bottom-to-surface connection installation between a plurality of undersea pipes resting on the sea bottom and a floating support on the surface comprising a hybrid tower constituted by a plurality of flexible pipes connected to rigid riser pipes, or vertical risers, with the top ends of said flexible pipes being secured to a turret pivoting freely in front of the vessel or within the vessel, generally in the front third of said vessel.
A large variety of bottom-to-surface connections are in existence that enable undersea well heads to be connected to an FPSO type floating support, and in certain oil field developments certain fields, a plurality of well heads are connected in parallel to a common bottom-to-surface connection so as to limit the number of pipes that are connected to the turret of the FPSO, thereby simplifying the design of the turret, which is designed mainly to take up the forces for anchoring the FPSO, which is itself subjected to the effects of swell, wind, and current.
Numerous configurations have been developed, and reference may be made to patent WO 2009/122098 in the name of the Applicant, which describes an FPSO fitted with such a turret and associated flexible pipes, more particularly for use in the extreme conditions that are to be encountered in the Arctic. Such a configuration is advantageous for medium depths of water, i.e. lying in the range 100 meters (m) to 350 m, or indeed in the range 500 m to 600 m. In particular, using flexible pipes over the full depth of the body of water between the rigid pipes resting on the sea bottom and the floating support allows the floating supports to move more than would be possible if rigid pipes were used. Nevertheless, with that type of bottom-to-surface connection between the turret of a floating support and pipes resting on the sea bottom, it is not possible to use said flexible pipes in a dipping catenary configuration, i.e. with a low point of inflection as described for hybrid tower type bottom-to-surface connections that comprise:                a vertical riser having its bottom end anchored to the sea bottom via a flexible hinge and connected to a said pipe resting on the sea bottom, and having its top end connected to a float immersed in the subsurface and serving to tension the riser; and        a connecting flexible pipe between the top end of said riser and a floating support on the surface, said connecting flexible pipe possibly taking up under the effect of its own weight the shape of a dipping catenary curve, i.e. a curve that goes down well below the float and subsequently rises up to said floating support, which dipping catenary is capable of accommodating large amounts of movement of the floating support, with this being absorbed by deformation of the flexible pipe, in particular by raising or lowering said low point of inflection of the dipping catenary.        
It should be recalled that the essential function of dipping flexible pipes is to absorb at least part of the movements of the top ends of the rigid pipes to which one of their ends is connected and/or the movements of the floating support to which their other end is connected, by mechanically decoupling the respective movements of the top ends of the rigid pipes to which they are connected from the movements of the floating supports to which they are also connected at their other ends.
In known manner, a said flexible connection pipe takes the shape of a dipping catenary curve under the effect of its own weight, i.e. it goes down well below its attachment points at each of its ends, respectively with the floating support and with the top end of the rigid pipe to which it is connected, providing the length of said flexible pipe is longer than the distance between its attachment point to the floating support and the top end of said rigid pipe to which it is connected.
In order to connect the flexible pipes to said rigid pipes or “risers”, gooseneck type devices known to the person skilled in the art are interposed between them, with an improved example of such a device being described in FR 2 809 136 in the name of the Applicant.
However, as soon as the water reaches a depth lying in the range 1000 m to 1500 m, or indeed 2000 m to 3000 m, the cost of such a multitude of flexible pipes becomes very high because of the developed length of each of said flexible pipes, since such flexible pipes are very complex and very difficult to fabricate if they are to achieve the levels of safety in operation that are required to enable them to remain in operation over periods of time that may reach or exceed 20 years to 25 years, or even more.
In particular, the flexible pipes run the risk of interfering with one another and striking against one another.
WO 2011/144864 describes a bottom-to-surface connection installation for a floating support having a turret to which the flexible pipes are fastened and secured via a guide structure. That type of bottom-to-surface connection is simultaneously compact, mechanically reliable in terms of being long-lasting, while also being relatively inexpensive and simple to make.
In WO 2011/144864, said guide structure is held in the subsurface between said turret and said carrier structure and it enables a plurality of dipping catenaries to be created that extend (concerning the center of the pipe) in planes that are substantially vertical and that intersect the vertical axis Z1Z1 of said guide structure, while also enabling said dipping catenaries to be spaced apart laterally from one another in a perpendicular plane that is horizontal.
Furthermore, the guide structure serves to guarantee the curvature of said dipping catenaries at their bottom points of inflection, ensuring that they always have a radius of curvature greater than a minimum radius of curvature beyond which deformation to the flexible pipe will become irreversible and/or damaging.
In all, said guide structure of WO 2011/144864 enables a larger number of flexible pipes to be used in optimized reduced space without those pipes interfering with one another and in particular without them striking one another, in the event of said floating support moving because of swell, current, and/or waves.
Nevertheless, in certain oil field developments, it is necessary to connect each of the well heads individually to said FPSO, which means that there are very many bottom-to-surface connections, thereby requiring the dimensions of the turret and/or of the guide structure as described in WO 2011/144864 to be increased in order to be capable of containing all of the flexible connections without them interfering with one another, and above all enabling the multiple pipe riser columns to be arranged so that any two said riser columns are spaced sufficiently far apart to avoid interfering with each other.